Kind of a late reply, but here is a way to do it en masse in linux.

Code: 
 find . -type f -name "*.rar" -execdir sh -c 'echo 0 >> "{}"' \;
It will find all the rar files below your current path and change the MD5.

You can also use it like this:

Code: 
 find /home/user/files/ -type f -name "*.rar" -execdir sh -c 'echo 0 >> "{}"' \;
That will find all rar files below /home/user/files/, and change the MD5 of all.

I love find, it's so cool

Anyway, hope that helps!